He says of the single-combinator combinatory algebra (call it X), Imagine that: a complete language report in two lines; a compiler that fits in a few K instead of Ms; no more arguments about smallness. I wonder... I have a sort of prejudice about combinatory algebra, that this sort of term rewriting system "doesn't count" as a programming language; the smallest sort of calculus that would count, I guess, is PCF, which is definitely a toy language, but it is really a programming language, with both bigstep (operational) and smallstep (computational) semantics.